EE382V: System-on-Chip (SoC) Design Lecture 12 EE382V: System-on-a-Chip (SoC) Design Lecture 12 – SoC Communication Architectures Source: Sudeep Pasricha (Colorado State), Nikil Dutt (UC Irvine) “On-Chip Communication Architectures”, Morgan Kaufmann, 2008 Andreas Gerstlauer Electrical and Computer Engineering University of Texas at Austin [email protected] Lecture 12: Outline • Introduction • Communication-centric design • Bus-based architectures • Topologies and structures • Decoding, arbitration, transfer modes • On-chip communication standards • AMBA and AXI • Networks-on-Chip (NoCs) • Topologies, switching, routing EE382V: SoC Design, Lecture 12 © 2014 A. Gerstlauer 2 © 2014 A. Gerstlauer 1 EE382V: System-on-Chip (SoC) Design Lecture 12 Technology Scaling Trends (1) • Total Interconnect Length on a Chip Highlights importance of interconnect design in future technologies EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 3 Technology Scaling Trends (2) • Relative delay comparison of wires vs. process technology Increasing wire delay limits achievable performance EE382V: SoC Design, Lecture 12 © 2008 Sudeep© 2014 Pasricha A. Gerstlauer & Nikil Dutt 4 © 2014 A. Gerstlauer 2 EE382V: System-on-Chip (SoC) Design Lecture 12 Communication Trumps Computation Core 1 SoCs Critical Decision Was uP Choice Core 2 Circa 2002 µP Core N Exploding core counts requiring more advanced Interconnects Main Bus EDA cannot solve this Sub architectural problem easily µP µP system Complexity too high to hand Mem Bus I/O Bus craft (and verify!) DRAMC SoCs Today Critical Decision Is Interconnect Choice Communication Architecture Design and Verification becoming Highest Priority in Contemporary SoC Design! Source: SONICS Inc. EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 5 Communication-Centric Design • Communication is THE most critical aspect affecting system performance • Communication architecture consumes upto 50% of total on-chip power • Ever increasing number of wires, repeaters, bus components (arbiters, bridges, decoders etc.) increases system cost • Communication architecture design, customization, exploration, verification and implementation takes up the largest chunk of a design cycle Communication architectures significantly affect performance, power, cost and time-to- market! EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 6 © 2014 A. Gerstlauer 3 EE382V: System-on-Chip (SoC) Design Lecture 12 On-Chip Communication Trends • Evolution of on-chip communication architectures EE382V: SoC Design, Lecture 12 © 2008 Sudeep© 2014 Pasricha A. Gerstlauer & Nikil Dutt 7 Bus-Based Architectures • Buses are the simplest and most widely used SoC interconnection networks • Bus: a collection of signals (wires) to which one or more IP components (which need to communicate data with each other) are connected • Only one component can transfer data on the shared bus at any given time Digital Input/ Micro- Signal Output Memory controller Processor Device Bus EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 8 © 2014 A. Gerstlauer 4 EE382V: System-on-Chip (SoC) Design Lecture 12 Bus Terminology EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 9 Bus Terminology • Master (or Initiator) • IP component that initiates a read or write data transfer • Slave (or Target) • IP component that does not initiate transfers and only responds to incoming transfer requests • Arbiter • Controls access to the shared bus • Uses arbitration scheme to select master to grant access to bus • Decoder • Determines which component a transfer is intended for • Bridge • Connects two busses • Acts as slave on one side and master on the other EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 10 © 2014 A. Gerstlauer 5 EE382V: System-on-Chip (SoC) Design Lecture 12 Bus Signal Lines address lines data lines control lines • A bus typically consists of three types of signal lines • Address – Carry address of destination for which transfer is initiated – Can be shared or separate for read, write data •Data – Carry information between source and destination components – Can be shared or separate for read, write data – Choice of data width critical for application performance • Control – Requests and acknowledgements – Specify more information about type of data transfer – Byte enable, burst size, cacheable/bufferable, write-back/through, … EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 11 Bus Physical Structure (1) • Tri-state buffer based bidirectional signals • Commonly used in off-chip/backplane buses • + take up fewer wires, smaller area footprint • - higher power consumption, higher delay, hard to debug EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 12 © 2014 A. Gerstlauer 6 EE382V: System-on-Chip (SoC) Design Lecture 12 Bus Physical Structure (2) • AND-OR based signals 13EE382V: SoC Design, Lecture 12 © 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt Bus Physical Structure (3) • MUX based signals • Separate read, write channels EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 14 © 2014 A. Gerstlauer 7 EE382V: System-on-Chip (SoC) Design Lecture 12 Bus Clocking • Synchronous Bus • Includes a clock in control lines • Fixed protocol for communication that is relative to clock • Involves very little logic and can run very fast • Require frequency converters across frequency domains EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 15 Bus Clocking • Asynchronous Bus • Not clocked • Requires a handshaking protocol – performance not as good as that of synchronous bus – No need for frequency converters, but does need extra lines • Does not suffer from clock skew like the synchronous bus EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 16 © 2014 A. Gerstlauer 8 EE382V: System-on-Chip (SoC) Design Lecture 12 Decoding and Arbitration • Decoding • Determines the target for any transfer initiated by a master • Arbitration • Decides which master can use the shared bus if more than one master request bus access simultaneously Decoding and Arbitration can either be • Centralized • Distributed EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 17 Centralized Decoding and Arbitration • Minimal change is required if new components are added to the system EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 18 © 2014 A. Gerstlauer 9 EE382V: System-on-Chip (SoC) Design Lecture 12 Distributed Decoding and Arbitration • + requires fewer signals compared to the centralized approach • - more hardware duplication, more logic/area, less scalable 19EE382V: SoC Design, Lecture 12 © 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt Arbitration Schemes (1) • Random • Randomly select master to grant bus access to • Static priority • Masters assigned static priorities • Higher priority master request always serviced first • Can be pre-emptive (AMBA2) or non-preemptive (AMBA3) • May lead to starvation of low priority masters • Round-robin • Masters allowed to access bus in a round-robin manner • No starvation – every master guaranteed bus access • Inefficient if masters have vastly different data injection rates • High latency for critical data streams EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 20 © 2014 A. Gerstlauer 10 EE382V: System-on-Chip (SoC) Design Lecture 12 Arbitration Schemes (2) • TDMA • Time division multiple access • Assign slots to masters based on BW requirements • If a master does not have anything to read/write during its time slots, leads to low performance • Choice of time slot length and number critical • Real-time worst-case latency guarantees (CAN bus) • TDMA/RR • Two-level scheme – If master does not need to utilize its time slot, second level RR scheme grants access to another waiting master • Better bus utilization • Higher implementation cost for scheme (more logic, area) EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 21 Arbitration Schemes (3) • Dynamic priority • Dynamically vary priority of master during application execution • Gives masters with higher injection rates a higher priority • Requires additional logic to analyze traffic at runtime • Adapts to changing data traffic profiles • High implementation cost (several registers to track priorities and traffic profiles) • Programmable priority • Simpler variant of dynamic priority scheme • Programmable register in arbiter allows software to change priority EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 22 © 2014 A. Gerstlauer 11 EE382V: System-on-Chip (SoC) Design Lecture 12 Bus Data Transfer Modes (1) • Single non-pipelined transfer • Simplest transfer mode – first request for access to bus from arbiter – on being granted access, set address and control signals – Send/receive data in subsequent cycles EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil Dutt 23 Bus Data Transfer Modes (2) • Pipelined transfer • Overlap address and data phases – Only works if separate address and data busses are present EE382V: SoC Design, Lecture 12© 2008 Sudeep © 2014 Pasricha A. Gerstlauer & Nikil